Sheet stacking apparatus and image forming apparatus

ABSTRACT

A sheet stacking apparatus includes: a sheet stacking apparatus comprising: a sheet stacking portion on which a sheet discharged from a sheet discharge port is stacked; a cover portion disposed above the sheet stacking portion and having a facing surface facing a sheet stacking surface of the sheet stacking portion; a driving portion configured to lift and lower the sheet stacking portion; and a controller configured to control the driving portion such that, when a sheet is not stacked on the sheet stacking portion, a distance between the facing surface of the cover portion and the sheet stacking surface of the sheet stacking portion is kept within a predetermined range, and when a sheet is stacked on the sheet stacking portion, a distance between the facing surface of the cover portion and an uppermost surface of the sheet stacked on the sheet stacking portion is kept within the predetermined range.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet stacking apparatus for stacking a sheet discharged from a sheet discharge port, and also relates to an image forming apparatus provided with the sheet stacking apparatus.

2. Description of the Related Art

Conventionally, an image forming apparatus such as a copying machine, a printer and a facsimile machine is equipped with a sheet stacking apparatus for stacking and receiving a sheet discharged from an image forming apparatus. The sheet stacked on a sheet stacking tray of the sheet stacking apparatus is kept on the tray until a user takes it out.

In recent years, the air conditioning at a work place or offices does not often rely on air conditioners alone, in view of energy saving. For example, windows are opened at offices and electric fans are used for the air circulation.

In such environment, an image forming apparatus may be situated near the windows, and the electric fan may be put near the image forming apparatus. In this case, the wind entering the offices from the open windows or the wind generated by the electric fan may blow off sheets stacked on the tray, or disturb the lined-up arrangement of the sheet bundle. The wind may disturb the sheets that are being discharged, and/or the sheets on the tray. Then, the order of the sheets discharged may be different from the order of the sheets stacked on the tray.

The image forming apparatus may be installed outside an office building. For example, one of the major installation places is a street, and a street stall has an image forming apparatus located inside the street stall. This is a copy-taking shop on the street. In such on-the-street copy-taking shop, the sheet(s) on the tray may fly with the wind, and further drop on the street. Then, the sheet(s) will become unclean.

In particular, a certain type of sheet such as a sheet having a small bias weight or thin paper is easily influenced by the wind. If the image forming apparatus is placed near an air blowing outlet of an air conditioner in a room, a lined-up arrangement of stacked thin sheets on the tray may be disturbed by the air blowing from the air conditioner.

In order to prevent the sheet(s) from falling off from the tray and scattering, Japanese Patent Laid-Open No. 2003-2513, for example, proposed one technique.

The technique of Japanese Patent Laid-Open No. 2003-2513 uses a press-down member that pushes the discharged sheet onto the upper face of the tray and causes the sheet to abut onto an apparatus main unit wall to achieve the lining up of the sheets.

However, if the discharged sheet should be pressed down on the upper surface of the tray by the press-down member, as in Japanese Patent Laid-Open No. 2003-2513, and the wind blows before the discharged sheet arrives at the press-down member, then the sheet may blow off from the tray.

As understood from the foregoing, the conventional image forming apparatus is not supposed to be used in a wind-blowing environment, such as where the wind blows around the image forming apparatus. As a result, the conventional image forming apparatus does not have a function to protect the sheet(s) stacked on the sheet stacking apparatus from the wind.

SUMMARY OF THE INVENTION

In view of the above problems, the present invention provides a sheet stacking apparatus that can prevent the blowing off of the sheet while the sheet is being discharged, can prevent the blowing off of the stacked sheet(s), can prevent the disturbing of the lined-up arrangement of the stacked sheets, and can prevent the order of sheets discharged from being different from the order of sheets stacked due to the wind.

A sheet stacking apparatus of the present invention includes: a sheet stacking apparatus comprising: a sheet stacking portion on which a sheet discharged from a sheet discharge port is stacked; a cover portion disposed above the sheet stacking portion and having a facing surface facing a sheet stacking surface of the sheet stacking portion; a driving portion configured to lift and lower the sheet stacking portion; and a controller configured to control the driving portion such that, when a sheet is not stacked on the sheet stacking portion, a distance between the facing surface of the cover portion and the sheet stacking surface of the sheet stacking portion is kept within a predetermined range, and when a sheet is stacked on the sheet stacking portion, a distance between the facing surface of the cover portion and an uppermost surface of the sheet stacked on the sheet stacking portion is kept within the predetermined range.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing a general structure of an image forming apparatus equipped with a sheet stacking apparatus.

FIG. 2 is a cross-sectional view of major components of the sheet stacking apparatus of the image forming apparatus.

FIG. 3 shows a control block diagram of the sheet stacking apparatus.

FIG. 4 is a cross-sectional view of major components of the sheet stacking apparatus when the wind blows against a discharge tray.

FIG. 5 is a cross-sectional view of major components of the sheet stacking apparatus when an upper cover is opened.

FIG. 6 is a perspective view of major components of the sheet stacking apparatus.

FIG. 7 is a flowchart showing the control for lifting and lowering the discharge tray.

FIG. 8 is a cross-sectional view of major components of the sheet stacking apparatus immediately after the sheet is discharged.

FIG. 9 is a cross-sectional view of major components of the sheet stacking apparatus when the discharge tray is lowered.

FIG. 10 illustrates a discharged sheet that is stacked on an uppermost face of a sheet bundle.

FIG. 11 is a cross-sectional view of major components of the sheet stacking apparatus immediately after the sheet is removed.

FIG. 12 is a perspective view of major components of the sheet stacking apparatus that has ventilation holes in the upper cover.

DESCRIPTION OF THE EMBODIMENTS

Now, an exemplary preferred embodiment of the present invention will be described in detail with reference to the drawings. It should be noted that dimensions, materials and shapes of elements and components described in the following embodiment and the relative positions between these elements and components may appropriately be changed and/or modified depending upon a configuration of an apparatus to which the present invention is applied and/or various conditions. Thus, the present invention is not limited to the dimensions, materials, shapes and the relative positions described in the following embodiment unless otherwise mentioned in an explicit manner.

Referring to FIG. 1, an image forming apparatus equipped with a sheet stacking apparatus according to this embodiment will be described. FIG. 1 is a schematic cross-sectional view showing a general structure of the image forming apparatus.

As illustrated in FIG. 1, the image forming apparatus 100 is a monochromatic high-speed printer that transfers a toner image formed on a photosensitive drum 101 onto a sheet P carried on a transfer belt 105 a.

Around the photosensitive drum 101, which is one example of an image bearing member, there are disposed a corona charger 102, an exposing device 103, a development device 104, a transfer unit 105, and a drum cleaning device 106. The photosensitive drum 101 has a photosensitive layer on an outer circumferential face of an aluminum cylinder, and rotates at a predetermined process speed in the direction of the arrow R1.

The corona charger 102 irradiates the photosensitive drum with a charged particle, which is generated upon corona discharge, such that the surface of the photosensitive drum 101 is uniformly charged with negative-polarity potential. The exposing device 103 scans a laser beam, which is prepared by ON-OFF modulating scanning line image data, with a rotating mirror. The scanning line image data is derived from an input image (prepared by developing). Then, the exposing device 103 writes an electrostatic image on the surface of the charged photosensitive drum 101. The development device 104 develops the electrostatic image formed on the photosensitive drum 101 to a toner image.

The transfer unit 105 forms a transfer portion T1 between the photosensitive drum 101 and a transfer belt 105 a. At the transfer portion T1, the toner image formed on the photosensitive drum 101 is transferred onto the sheet P carried on the transfer belt 105 a.

The sheets P stored in a recording material cassette 110 are conveyed, sheet by sheet, to resist rollers 120 by separation roller 111. The resist rollers 120 receive the sheet P and hold it at the deactivated state, and feeds the sheet P to the transfer portion T1 in synchronization with the preparation timing of the toner image on the photosensitive drum 101.

The drum cleaning device 106 causes a cleaning blade to slidably contact the photosensitive drum 101, and recovers the residual toner remaining on the photosensitive drum, which are not transferred to the sheet P.

Upon applying a positive-polarity DC voltage to the transfer unit 105, the toner image carried (born) on the photosensitive drum 101 is born by the transfer belt 105 a and transferred to the sheet P that passes through the transfer portion T1.

The sheet P, on which the toner image is transferred, is self-stripped from the transfer belt 105 a and conveyed to a fixing device 107. Thus, the sheet P is thermally pressed by the fixing device 107, and the toner image is heat-fixed on the surface of the sheet. Subsequently, the sheet P is discharged to the sheet stacking tray 300 of the sheet stacking apparatus from the apparatus main unit housing 100K by the discharge rollers 113.

The image forming apparatus 100 has a duplex image-forming function (both sides image-forming function). When images are formed on both sides of the sheet, the fixing device 107 finishes the fixing operation to the first side of the sheet P and then conveys the sheet P to the image forming portion again through a reversed convey path 114. A toner image is formed on the second side of the sheet P, which is re-fed to the image forming portion, by the same image forming process as described above.

Now, the sheet stacking apparatus of this embodiment will be described in detail with reference to FIG. 2 to FIG. 12. FIG. 2 is a cross-sectional view of major components showing a general structure of the sheet stacking apparatus in the image forming apparatus.

In the sheet stacking apparatus shown in FIG. 2, the posture of the sheet stacking tray 300 has different vertical heights with respect to the sheet conveying direction (sheet discharging direction). The height of the sheet stacking tray 300, measured in the gravity direction, at an upstream position in the sheet conveying direction is different from the height of the sheet stacking tray 300 at a downstream position. By having the changing height in the sheet conveying direction, the discharged sheet slides on the upper surface of the tray or an upper surface of another sheet already stacked on the tray in a downward direction by the force of gravity when the discharged sheet falls on the sheet stacking tray. By providing a stopper, such as a wall, which abuts onto the sliding sheet(s), at the lower side, the sheets are collected at the stopper and properly stacked in order. In this embodiment, the sheet stacking tray 300 takes a posture, with its upstream side in the sheet discharging direction being lower than its downstream side. Thus, the discharged sheets P are collected on the side of the apparatus main unit wall 115 that has the sheet discharge port 117.

Above the sheet stacking tray 300, there is provided an upper cover 301, which is a cover portion, having a facing plane (a facing surface) facing the sheet stacking surface of the sheet stacking tray 300. That face of the upper cover 301 which faces the sheet stacking tray (facing plane) serves as a guide plane for guiding the sheet such that paper jamming does not occur and the sheet is not damaged even if the sheet discharged from the sheet discharge port 117 contacts the upper cover.

The sheet stacking tray 300 is equipped with a lower detection sensor 307 and a lower light-shielding plate 307 a. The lower detection sensor 307 has a photo-interrupter and detects presence/absence of the sheet(s) stacked on the sheet stacking tray 300. The lower light-shielding plate 307 a rotates as it is pushed by the stacked sheet(s). Then, the lower light-shielding plate 307 a shields the light between the light-emitting portion and the light-receiving portion of the lower detection sensor 307. When the lower light-shielding plate 307 a shields the light between the light emitting portion and the light receiving portion of the lower detection sensor 307, the lower detection sensor (first detection sensor) 307 generates a detection signal indicating the presence of the sheet. In order to detect the height of the sheet stacking surface of the sheet stacking tray 300 or the uppermost face of the sheet(s) stacked on the sheet stacking tray 300, the upper cover 301 is equipped with an upper detection sensor 302 that has a photo-interrupter. The upper cover 301 is also equipped with an upper light-shielding plate 302 a. The upper light-shielding plate 302 a abuts onto the sheet stacking surface of the sheet stacking tray 300 or the uppermost face of the sheet(s) stacked on the sheet stacking tray 300. When the upper light-shielding plate reaches a predetermined height, the upper light-shielding plates shields the light between the light emitting portion and the light receiving portion of the upper detection sensor 302, and causes the upper detection sensor (second detection sensor) 302 to alter its output signal. As shown in the block diagram of FIG. 3, the lower detection sensor 307 and the upper detection sensor 302 are connected to a controlling portion (controller) 305 provided in the image forming apparatus. The sheet stacking tray 300 is equipped with a driving unit (driving portion) 306, including gears, a motor M and other components, for lifting and lowering the sheet stacking tray 300 in the height direction of sheet stacking. The controlling portion 305 controls the driving unit 306 based on the detection signals from the lower detection sensor 307 and upper detection sensor 302.

When no sheet is stacked on the sheet stacking tray 300, the controlling portion 305 controls the driving unit 306 such that the distance between the sheet stacking surface of the sheet stacking tray 300 and the guide plane of the upper cover 301 is maintained at the predetermined gap A (FIG. 2). In this embodiment, the predetermined gap A is 5 mm. Because the gap A is such small gap, the air resistance between the upper cover 301 and the sheet stacking tray 300 is large and the pressure loss is large when the wind blows against the sheet stacking tray. Therefore, the wind does not enter the gap between the upper cover 301 and the sheet stacking tray 300, and flows to the less-resistant areas, i.e., around the sheet stacking tray 300 and the upper cover 301 (FIG. 4). It should be noted that the positional relationship between the sheet stacking surface of the sheet stacking tray 300 and the guide face of the upper cover 301 is not necessarily in parallel to each other (FIG. 2). Experiments revealed that the wind blowing toward the tray entered the gap between the upper cover 301 and the sheet stacking tray 300 and the stacked sheets were disturbed when the gap A was 7 mm. When the gap A was reduced to 6 mm and the wind blew toward the tray, an amount of wind entering the gap between the upper cover and the sheet stacking tray decreased, but the stacked sheets were sometimes disturbed. When the gap A was reduced to 5 mm or less and the wind blew toward the tray, the wind did not enter the gap between the upper cover and the sheet stacking tray, and the stacked sheets were never disturbed by the wind. Consequently, the gap A is preferably equal to or smaller than 5 mm. It should be noted that if the gap A is equal to or smaller than 3 mm, the jamming may occur when the sheet is discharged to the sheet stacking tray 300. In view of this, the minimum gap between the sheet stacking surface of the sheet stacking tray 300 and the guide face of the upper cover 301 is set to a value that can ensure satisfactory discharging of the sheets. As such, the gap A can take an appropriate value in a predetermined range, and is not limited to the above-mentioned exemplary value(s). It was confirmed by experiments that satisfactory discharging of the sheets was achieved as long as the gap A between the upper face of the top sheet of the stacked sheets on the sheet stacking tray 300 and the guide face of the upper cover 301 was set to the above-mentioned dimension(s) even when the sheets were stacked on the sheet stacking tray 300 (FIG. 9).

The upper cover 301 is attached to the apparatus main unit housing 100K and pivotable (openable and closable) relative to the apparatus main unit housing 100K such that the upper cover 301 opens upward with respect to the sheet stacking tray 300. The upper cover 301 can move to the closed position (FIG. 4) and the open position (FIG. 5). At the closed position, that face of the upper cover 301 which is designed to face the sheet stacking surface of the sheet stacking tray 300 extends over the sheet stacking surface of the sheet stacking tray 300. At the open position, the upper cover 301 allows the picking up (taking out) of the sheet P (or sheet bundle) stacked on the sheet stacking tray 300. Rotation shaft 309 provided at both ends of the upper cover 301 in the width direction perpendicular to the sheet discharging direction engage with support portions 116 provided on the apparatus main unit housing 100K above the sheet discharge port 117 as shown in FIG. 6, and therefore the upper cover 301 is supported such that it can rotate (pivot). When the upper cover 301 is at the closed position shown in FIG. 2, stopper 308 provided at both ends of the upper cover 301 abut onto the apparatus main unit wall 115 and regulates the position of the upper cover 301.

On the other hand, the gap B between the upstream end 303 of the upper cover 301 in the sheet discharging direction and the apparatus main unit wall 115, as shown in FIG. 2, is set to be equal to or smaller than a predetermined value. If the gap B is greater than the predetermined value and the wind blows in a certain direction, the front end of the sheet P discharged from the sheet discharge port 117 may take an awkward behavior before the sheet front end reaches the upper cover 301, and the sheet may be discharged between the upper cover 301 and the sheet discharge port 117. In addition, when the gap B is greater than the predetermined value, the rear end of the sheet P stacked on the sheet stacking tray 300 may be rolled up by the wind, and therefore the front end of a next discharged sheet P may slip under the rear end of the rolled up sheet P. In consideration of experimental results, the gap B was set to 10 mm in this embodiment regardless of the sheet size.

Now, the movements of the sheet stacking tray relative to the upper cover situated at the closed position will be described with reference to the flowchart shown in FIG. 7.

As shown in FIG. 8, as the copying operation starts and the sheet is stacked on the sheet stacking tray 300 (S101), the detection signal of the lower detection sensor 307 becomes OFF from ON (S102). The ON state is shown in FIG. 2. When several tens of sheets are stacked and the height of the bundle of sheets P (height of the uppermost surface of the sheet bundle) reaches a predetermined height (L in FIG. 8) which does not close (block) the sheet discharge port 117, then the detection signal of the upper detection sensor 302 becomes OFF (YES at S103). When such detection signals are introduced to the controlling portion 305 as shown in the block diagram of FIG. 3, the controlling portion 305 controls the driving portion 306 to cause the sheet stacking tray 300 to lower (S104). When the sheet stacking tray 300 starts lowering in the arrow D direction and arrives at a position that causes the detection signal of the upper detection sensor 302 to become ON (S105), i.e., the position of the gap A, then the controlling portion 305 controls the driving portion 306 to stop the lowering of the sheet stacking tray 300 (S106; FIG. 9). In this manner, the gap A between the sheet stacking surface of the sheet stacking tray 300 and the facing plane of the upper cover 301 is maintained in the predetermined range, and therefore the next discharged sheet P does not push out the sheets P′ already stacked on the sheet stacking tray 300 and is stacked on the sheets P′ (FIG. 10).

When the sheets P′ stacked on the sheet stacking tray 300 are removed (S107; YES), the detection signal of the lower detection sensor 307 becomes ON (S108), and the controlling portion 305 controls the driving portion 306 to cause the sheet stacking tray 300 to lift in the arrow U direction (S109; FIG. 11). When the detection signal of the upper detection sensor 302 is turned to OFF by the sheet stacking surface of the sheet stacking tray 300 (5110), the controlling portion 305 controls the driving portion 306 to cause the elevated sheet stacking tray 300 to lower in the arrow D direction (S111). When the detection signal of the upper detection sensor 302 becomes ON at the position of the predetermined gap A (S112), the controlling portion 305 controls the driving portion 306 again to stop the lowering of the sheet stacking tray 300 (S113). As a result, the sheet stacking tray 300 is brought to the initial position (stand-by position).

In this embodiment, as described above, the gap A between the sheet stacking surface of the sheet stacking tray 300 and the facing plane of the upper cover 301 is maintained in the predetermined range when no sheet is stacked on the sheet stacking tray 300. When the sheet or sheets are stacked on the sheet stacking tray, the gap A between the uppermost face of the sheet(s) stacked on the sheet stacking tray 300 and the facing plane of the upper cover 301 is maintained in the predetermined range. As a result, it is possible to prevent the blowing off of the sheet(s) being discharged from the sheet discharge port by the wind, the blowing off of the sheet(s) stacked on the sheet stacking tray by the wind, and the disturbing of the lining up of the sheets on the sheet stacking tray by the wind. It is also possible to prevent the disturbing of the discharged order of the sheet(s) and the stacked order of the sheets by the wind. Because the gap B between the upstream end 303 of the upper cover 301 in the sheet discharging direction and the apparatus main unit wall 115 which has the sheet discharge port 117 is equal to or smaller than the predetermined value, it is possible prevent the blowing off of the sheet(s) being discharged from the sheet discharge port by the wind and the blowing off of the sheet(s) stacked on the sheet stacking tray by the wind.

It should be noted that the following structure may be added to the above-described exemplary embodiment if necessary. When the sheets are stacked on the sheet stacking tray 300, heat discharged from the sheet bundle P′ is trapped (confined) in a clearance region between the upper cover 301 and the sheet bundle. As a result, dew may condense on the guide plane of the upper cover 301, and/or the sheet bundle P′ may become difficult to cool. To deal with this, a ventilation hole (or holes) 304 may be formed in the upper cover 301 for upwardly ventilating the heat generated from the sheet(s) as long as the ventilation holes 304 do not decrease the above-described air resistance (FIG. 12). Specifically, the upper cover 301 has one or more ventilation holes 304 that penetrate the upper cover from the facing plane in the gravity direction. Open side gaps between the sheet stacking tray 300 and the upper cover 301 are also used for ventilation.

Although the image forming apparatus is a copying machine in the above-described embodiment, the present invention is not limited in this regard. For example, the image forming apparatus may be other types of image forming apparatus such as a printer and a facsimile machine, or yet other types of image forming apparatus such as a multi-function printer that has the combined functions of the printer and facsimile machine. By applying the present invention to a sheet stacking apparatus of such image forming apparatus, the same advantages as those described in the foregoing are obtained.

Although the sheet stacking apparatus is integral with the image forming apparatus in the above-described embodiment, the present invention is not limited in this regard. For example, the sheet stacking apparatus may be removable from and attachable to the image forming apparatus. By applying the present invention to such sheet stacking apparatus, the same advantages as those described above are obtained.

Although the sheet stacking apparatus is used for the image forming apparatus in the above-described embodiment, the present invention is not limited in this regard. For example, the sheet stacking apparatus of the present invention may be used for a sheet processing apparatus, such as a finisher, that can selectively perform the aligning (lining up) process to the sheets and the stapling (or binding, filing) process to the sheets. In this case, the same advantages as those described above are obtained.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2013-090539, filed Apr. 23, 2013, which is hereby incorporated by reference herein in its entirety. 

What is claimed is:
 1. A sheet stacking apparatus comprising: a sheet stacking portion on which a sheet discharged from a sheet discharge port is stacked; a cover portion disposed above the sheet stacking portion and having a facing surface facing a sheet stacking surface of the sheet stacking portion; a driving portion configured to lift and lower the sheet stacking portion; and a controller configured to control the driving portion such that, when a sheet is not stacked on the sheet stacking portion, a distance between the facing surface of the cover portion and the sheet stacking surface of the sheet stacking portion is kept within a predetermined range, and when a sheet is stacked on the sheet stacking portion, a distance between the facing surface of the cover portion and an uppermost surface of the sheet stacked on the sheet stacking portion is kept within the predetermined range.
 2. The sheet stacking apparatus further comprising: a first detection sensor disposed at the sheet stacking portion configured to generate a signal based on presence or absence of a sheet stacked on the sheet stacking portion; and a second detection sensor disposed at the cover portion configured to generate a signal based on a position of a sheet stacking surface of the sheet stacking portion or a position of an uppermost surface of a sheet stacked on the sheet stacking portion, wherein the controller controls the driving portion according to detection signals from the first detection sensor and the second detection sensor.
 3. The sheet stacking apparatus according to claim 1, wherein the cover portion is openablly disposed such that the cover portion opens upwardly with respect to the sheet stacking portion.
 4. The sheet stacking apparatus according to claim 3, wherein there is a gap between an upstream end of the cover portion in a sheet discharging direction and an apparatus main unit wall which has the sheet discharge port.
 5. The sheet stacking apparatus according to claim 1, wherein the cover portion has one or a plurality of holes that penetrate the cover portion from the facing surface in a gravity direction.
 6. The sheet stacking apparatus according to claim 1, wherein the facing surface of the cover portion functions as a guide surface guiding a sheet discharged from the sheet discharge port.
 7. An image forming apparatus comprising: an image forming device for forming an image on a sheet; and a sheet stacking apparatus for stacking the sheet on which the image is formed, the sheet stacking apparatus including: a sheet stacking portion on which the sheet discharged from a sheet discharge port is stacking; a cover portion disposed above the sheet stacking portion and having a facing surface facing a sheet stacking surface of the sheet stacking portion; a driving portion configured to lift and lower the sheet stacking portion; and a controller for controlling the driving portion, wherein the controller controls the driving portion such that, when a sheet is not stacked on the sheet stacking portion, a distance between the facing surface of the cover portion and the sheet stacking surface of the sheet stacking portion is kept within a predetermined range, and when a sheet is stacked on the sheet stacking portion, a distance between the facing surface of the cover portion and an uppermost surface of the sheet stacked on the sheet stacking portion is kept within the predetermined range.
 8. The image forming apparatus according to claim 7 further comprising: a first detection sensor disposed at the sheet stacking portion configured to generate a signal based on presence or absence of a sheet stacked on the sheet stacking portion; and a second detection sensor disposed at the cover portion configured to generate a signal based on a position of a sheet stacking surface of the sheet stacking portion or a position of an uppermost surface of a sheet stacked on the sheet stacking portion, wherein the controller controls the driving portion according to detection signals from the first detection sensor and the second detection sensor.
 9. The image forming apparatus according to claim 6, wherein the cover portion is openablly disposed such that the cover portion opens upwardly with respect to the sheet stacking portion.
 10. The image forming apparatus according to claim 9, wherein there is a gap between an upstream end of the cover portion in a sheet discharging direction and an apparatus main unit wall which has the sheet discharge port.
 11. The image forming apparatus according to claim 7, wherein the cover portion has one or a plurality of holes that penetrate the cover portion from the facing surface in a gravity direction.
 12. The image forming apparatus according to claim 7, wherein the facing surface of the cover portion functions as a guide surface guiding a sheet discharged from the sheet discharge port. 